This invention relates generally to magnetic resonance imaging (MRI), and more particularly, to decoupling radio frequency (RF) detector arrays used for MRI.
Generally, MRI is a well-known imaging technique. A conventional MRI device establishes a homogenous magnetic field, for example, along an axis of a person""s body that is to undergo MRI. This homogeneous magnetic field conditions the interior of the person""s body for imaging by aligning the nuclear spins of nuclei (in atoms and molecules forming the body tissue) along the axis of the magnetic field. If the orientation of the nuclear spin is perturbed out of alignment with the magnetic field, the nuclei attempt to realign their nuclear spins with an axis of the magnetic field. Perturbation of the orientation of nuclear spins may be caused by application of radio frequency (RF) pulses. During the realignment process, the nuclei precess about the axis of the magnetic field and emit electromagnetic signals that may be detected by one or more coils placed on or about the person.
The frequency of the magnetic resonance (MR) signal emitted by a given precessing nucleus depends on the strength of the magnetic field at the nucleus"" location. As is well known in the art, it is possible to distinguish radiation originating from different locations within the person""s body by applying a field gradient to the magnetic field across the person""s body. For the sake of convenience, direction of this field gradient may be referred to as the left-to-right direction. Radiation of a particular frequency may be assumed to originate at a given position within the field gradient, and hence at a given left-to-right position within the person""s body. The application of such a field gradient is also referred to as frequency encoding.
However, the application of a field gradient does not allow for two-dimensional resolution, since all nuclei at a given left-to-right position experience the same field strength, and hence emit radiation of the same frequency. Accordingly, the application of a frequency-encoding gradient, by itself, does not make it possible to discern radiation originating from the top versus radiation originating from the bottom of the person at a given left-to-right position. Resolution has been found to be possible in this second direction by application of gradients of varied strength in a perpendicular direction to thereby perturb the nuclei in varied amounts. The application of such additional gradients is also referred to as phase encoding.
Frequency-encoded data sensed by the coils during a phase encoding step is stored as a line of data in a data matrix known as the k-space matrix. Multiple phase encoding steps are performed in order to fill the multiple lines of the k-space matrix. An image may be generated from this matrix by performing a Fourier transformation of the matrix to convert this frequency information to spatial information representing the distribution of nuclear spins or density of nuclei of the image material.
Imaging time is largely a factor of desired signal-to-noise ratio (SNR) and the speed with which the MRI device can fill the k-space matrix. In conventional MRI, the k-space matrix is filled one line at a time. Although many improvements have been made in this general area, the speed with which the k-space matrix may be filled is limited. To overcome these inherent limits, several techniques have been developed to simultaneously acquire multiple lines of data for each application of a magnetic field gradient. These techniques, which may collectively be characterized as xe2x80x9cparallel imaging techniquesxe2x80x9d, use spatial information from arrays of RF detector coils to substitute for the encoding which would otherwise have to be obtained in a sequential fashion using field gradients and RF pulses. The use of multiple effective detectors has been shown to multiply imaging speed, without increasing gradient switching rates or RF power deposition.
Two such parallel imaging techniques that have recently been developed and applied to in vivo imaging are SENSE (SENSitivity Encoding) and SMASH (simultaneous acquisition of spatial harmonics). Both techniques include the parallel use of a plurality of separate receiving elements, with each element having a different respective sensitivity profile, and combination of the respective spin resonance signals detected enables a reduction of the acquisition time required for an image (in comparison with conventional Fourier image reconstruction) by a factor which in the most favorable case equals the number of the receiving members used (see Pruessmann et al., Magnetic Resonance in Medicine Vol. 42, p.952-962, 1999).
A drawback of the SENSE technique, for example, results when the component coil sensitivities are either insufficiently well characterized or insufficiently distinct from one another. These instabilities may manifest as localized artifacts in the reconstructed image, or may result in degraded signal to noise ratio (SNR). Accordingly, it is desirable to implement RF coil arrays in MRI systems that (among other aspects) provide increased SNR with or without the use of parallel imaging techniques such as SENSE.
Additionally, image artifacts are also attributable due to the mutual couplings between coils in a cluster of closely situated surface coils, which have been separately tuned and matched. The mutual couplings between the coils generate coupled modes, which cause splitting in the coils"" resonant spectrum. Consequently, the coils become detuned and mismatched, causing reductions in the SNR. To sustain the SNR of the coils and avoid image artifacts caused by coil coupling, some decoupling mechanisms are needed to degenerate the multiple coupled modes into a single mode that resonates at the MR frequency.
In a typical multiple coil array arrangement, several adjacent coils are provided for receiving signals during imaging. To limit or reduce a common problem of cross talk between adjacent coils, generally adjacent coils are overlapped and a low impedance preamplifier is used for the coils not contained within an overlapping pair. Due to the current-carrying paths established by each coil of the array, such overlapping and preamplifier configuration reduces and/or cancels mutual inductive coupling between the coils, thereby reducing cross talk.
Most recently, parallel spatial encoding techniques such as SMASH and SENSE and the like impose a new design criterion that the complex sensitivities of the phased array coils should be sufficiently orthogonal, or alternatively sufficiently distinct from one another. Conventional overlapping coil and preamplifier arrangements do not generally meet this requirement. Thus, there is a need for a method and apparatus for decoupling RF detector arrays for use in parallel imaging using MRI.
In a first aspect, a radio frequency (RF) detector array assembly for use in a magnetic resonance imaging (MRI) system is provided. The RF detector array assembly comprises at least one array of RF detectors, wherein the array has a plurality of RF detector elements for use in acquiring radio frequency (RF) signals from the MRI system, and, a decoupling interface coupled to each of the plurality of detector elements for decoupling each detector element from the remaining detector elements.
In a second aspect, a method for decoupling radio frequency (RF) detector array elements in a magnetic resonance imaging (MRI) system is provided. The method comprises the steps of providing at least one RF detector array, wherein the detector array has a plurality of RF detector elements, and, providing a decoupling interface coupled to each of the plurality of detector elements for decoupling each detector element from the remaining detector elements.